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Marco L. Carrera
,
John R. Gyakum
, and
Da-Lin Zhang

Abstract

Secondary cyclogenesis has been identified as a difficult forecast challenge. In this paper, the authors examine the dominant physical processes associated with the predictability of a case of explosive secondary marine cyclogenesis and provide a better understanding of the large variability in the recent model-intercomparison simulations of the case. A series of sensitivity experiments, involving changes to the model initial conditions and physical parameterizations, is performed using the Canadian Mesoscale Compressible Community Model with a grid size of 50 km.

It is found that errors in the model initial conditions tend to decay with time, and more rapidly so in “dry” simulations. The model fails to produce the secondary cyclogenesis in the absence of latent heating. Water vapor budget calculations from the control experiment show that the surface moisture flux from 6 to 12 h is the largest contributor of water vapor to the budget area in the vicinity of the cyclone center, and remains an important moisture supply throughout the integration period. During the first 12 h, these fluxes are crucial in inducing grid-scale diabatic heating and destabilizing the lower troposphere, thereby facilitating the subsequent rapid deepening of the storm. A secondary maximum in surface latent heat flux to the north and east of the primary maximum acts to force the cyclogenesis event to the south and east of a coastal circulation center. When the surface evaporation is not allowed, much less precipitation is produced and the secondary cyclone fails to develop. Calculations of the potential temperature on the dynamic tropopause (i.e., 2-PVU surface) in the absence of surface evaporation indicate a significantly damped thermal wave when compared with the control integration.

This result for a case of secondary cyclogenesis differs from those generally found for large-scale extratropical cyclogenesis where upper-level baroclinic forcings tend to dominate, and motivates the need for better physical parameterizations, including the condensation and boundary layer processes, in operational models. The authors speculate that the different treatment of condensation and boundary layer processes may have been partly responsible for the enhanced variability in the simulation of this case in a recently completed international mesoscale model intercomparison experiment.

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Christophe Lavaysse
,
Marco Carrera
,
Stéphane Bélair
,
Normand Gagnon
,
Ronald Frenette
,
Martin Charron
, and
M. K. Yau

Abstract

The aim of this study is to assess the impact of uncertainties in surface parameter and initial conditions on numerical prediction with the Canadian Regional Ensemble Prediction System (REPS). As part of this study, the Canadian version of the Interactions between Soil–Biosphere–Atmosphere (ISBA) land surface scheme has been coupled to Environment Canada’s numerical weather prediction model within the REPS. For 20 summer periods in 2009, stochastic perturbations of surface parameters have been generated in several experiments. Each experiment corresponds to 20 simulations differing by the perturbations at the initial time of one or several surface parameters or prognostic variables. The sensitivity to these perturbations is quantified especially for 2-m temperature, 10-m wind speed, cloud fraction, and precipitation up to 48-h lead time. Spatial variability of these sensitivities over the North American continent shows that soil moisture, albedo, leaf area index, and SST have the largest impacts on the screen-level variables. The temporal evolution of these sensitivities appears to be closely linked to the diurnal cycle of the boundary layer. The surface perturbations are shown to increase the ensemble spread of the REPS for all screen-level variables especially for 2-m temperature and 10-m wind speed during daytime. A preliminary study of the impact on the ensemble forecast has shown that the inclusion of the surface perturbations tends to significantly increase the 2-m temperature and 10-m wind speed skill.

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